1. Field of the Invention
The present invention relates generally to mixer circuits and more particularly to a single side band mixer incorporating digital logic switching elements and field effect transistors (FETs) in order to accomplish signal mixing over a wide frequency range including direct current (DC) levels.
2. Description of the Prior Art
A mixer is an electronic circuit that performs frequency conversion by multiplying two signals, a function that is applicable to many areas of radio and other high frequency communications systems. The most prevalent use for a mixer circuit is in a radio circuit to obtain an intermediate frequency (IF) signal by multiplying a radio frequency (RF) signal by a local oscillator (LO) signal. The simplest mixer circuit uses a single diode, but for increased performance more sophisticated circuits are often used.
For high frequency applications, the two main types of mixers are switching mixers and nonlinear mixers. Switching mixers include both single balanced and double balanced types, and nonlinear mixers use a nonlinear device such as a schottky diode, field effect transistor (FET) or other transistor to perform the multiplication function. Switching mixers also require a nonlinear device for performing the mixing function. An advantage of the double balanced mixer is that it can isolate the two input signals from each other and from the output signal, thus allowing the bandwidths of the two input signals to overlap without producing noise in the form of spurious signal artifacts.
A mixer circuit has three terminals that in general use are identified as the radio frequency (RF) terminal, the local oscillator terminal (LO), and the intermediate frequency terminal (IF). The RF terminal is where the high frequency signal is applied when the desired output will be the IF signal at a lower frequency, also known as a downconverted signal, or where the high-frequency signal is output when an IF input signal is upconverted. The LO signal is generally the stronger input signal, and is used to modulate the FETs with a switching action that effectively reverses the path of the signal between the other two terminals. In the case of a downconverter, the IF terminal is where the RF signal that was modified by the LO signal is passed. In the case of an upconverter, the IF terminal and the LO terminal have the input signals that are multiplied to produce the higher frequency RF output signal.
As stated hereinabove, a mixer may be used to convert a signal down in frequency (as in a radio receiver) or up in frequency. When two sine waves are multiplied or beat against each other, the result includes both sum and difference frequencies. This can be expressed as:sin(ω1t)×sin(ω2t)=½{(cos [(ω1−ω2)t]−cos [(ω1+ω2)t]}In a single sideband (SSB) mixer, either the sum or the difference frequency will be used but not both. Efficient and complete removal of the unused frequency is necessary to prevent noise. A high-side mixer is one in which the LO is higher in frequency than the RF, a low-side mixer is one in which the LO is lower in frequency. When an SSB mixer is used in a downconverter or most receiver circuits, the conversion loss is the difference in amplitude of the available RF signal to the IF signal output. In an upconverter application, the conversion gain is the difference in amplitude from the IF signal to the RF signal.
Since the operation of a mixer depends on a nonlinear device, there are unavoidably some intermodulation products generated along with the desired output signal. Of particular concern are the third order intermodulation products, which in a receiver circuit are close in frequency to the fundamental IF frequency and thus are difficult to filter out. These products affect the maximum dynamic range for the mixer. The measurement used to define this maximum dynamic range is taken to be the point on a curve of the RF input versus the IF output where the desired output signal and the third order intermodulation products have equal amplitude as the RF input is raised. This point at which the power of the fundamental IF signal equals the power of the third order product is called the third order intercept point or IP3, and input IP3 is used as an important figure of merit for the quality of a mixer circuit. The output third order intercept point is defined as the difference between the input IP3 and the conversion loss, where higher conversion losses result in a lower output IP3.
An example of a double-balanced single sideband mixer with a high IP3 is taught by U.S. Pat. No. 6,957,055 entitled DOUBLE BALANCED FET MIXER WITH HIGH IP3 AND IF RESPONSE DOWN TO DC LEVELS. This circuit uses FETs as nonlinear elements to accomplish the mixer function, but also incorporates several balun transformers to allow a signal path down to DC. Having the circuit's IF frequency response reaching down to DC allows the mixer to be used in in-phase quadrature (I/Q) modulators, which are useful in circuits including certain types of phase-locked loops (PLLs).